Devices using various types of media such as optical disks, magnetic optical disks, and a flexible magnetic disk are known in the art as disk drives. Among them, hard disk drives (hereinafter referred to as HDDs) have become popular as storage devices for computers to such an extent that they are one of the storage devices indispensable for modern computer systems. Further, not limited to the computers as described above, HDDs are expanding more and more in application due to their excellent properties. For example, HDDs are used for moving picture recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
A magnetic disk used in the HDD has a plurality of data tracks that are concentrically formed. A plurality of pieces of servo data each having address information, and a plurality of data sectors each including user data, are written to each data track. A plurality of data sectors are written between servo data areas. By accessing, according to the address information of the servo data, a desired data sector by use of a head element of a head slider supported by an actuator that pivotally moves, it is possible to write data to the data sector or to read data from the data sector.
In order to improve the recording density of a magnetic disk, it is important to reduce the clearance between the magnetic disk and a head element flying above the magnetic disk. For this reason, some mechanisms for adjusting the clearance are proposed. As one example of the mechanisms, a head slider includes a heater, and the clearance is adjusted by heating a head element by use of the heater. In this specification, this is called TFC (Thermal Fly height Control). According to the TFC, an electric current is supplied to a heater to generate heat, which causes a head element to protrude. This makes it possible to reduce the clearance between a magnetic disk and the head element. The TFC is disclosed in, for example, Japanese Patent Laid-Open No. 2004-342151 (patent document 1). Patent document 1 discloses that in order to prevent crosstalk from causing noises in a read signal, tilts of the rising and falling edges of an electric current supplied to a heater are reduced.
If the TFC is adopted, a heater is repeatedly turned ON/OFF several trillions of times during the use of the HDD. Because of it, the long-term reliability of the TFC becomes a major concern. As a result of the study of the inventors, it has been found that if a heater is repeatedly turned ON/OFF by TFC, the structural stress is applied to the heater and the head element, which may cause a device breakdown. The thermal expansion and thermal shrinkage cause metal fatigue of materials of the heater. As a result, the breakage of the heater, and a short circuit by electromigration, may occur. In addition, in the boundary between the heater and a surrounding member thereof, a crack may occur due to the difference in material between them. In another case, because a read element and a write element repeatedly expand and shrink, a crack occurs in such part, or the electromigration is accelerated.
As one of methods for reducing the structural stress applied to the heater and the head element, it is thought that a slew rate of the output to the heater element from a preamplifier (the speed of the rising edge/falling edge of a signal waveform) is decreased. However, because the useless decrease in slew rate causes the response speed of the clearance adjustment to decrease, it is thought that the intended purpose of the TFC cannot be achieved depending on processing conditions.